I've played though both and ended up going with the Demeter. I liked its warmth. It's also got a mild, natural compression that I didn't notice as much on the Kern. The Kern seemed to have a more aggressive EQ section than the Demeter, but I thought the Kern was overpriced.
There both great pre's though. I think once you get to the level of quality exemplified by those 2 companies, it's purely a matter of preference. Hope that helps.

I like my Kern better than the Aguilar, felt its sound was richer and more natural. The EQ section is simple but lets you get the sound that you want, and the Kern has a standby switch. Haven't played the Demeter.

I own a Kern. I have played a Demeter about 2 years ago in a store. I believe they have changed since then. I played the Aguilar in a store about 1 year ago.

Kern: I have commented on this recently. Look for the thread on Kern. Love the "Tonal Balance" control. I have one with the wall-wart. PITA if you don't have a space to mount it in your rack. Newer versions apparently don't have this. (Or so I have heard.)

Haven't tried Aguilar yet but own a Demeter and tried a Kern in a guitar shop. Both excellent! I agree that the Kern seemed to have a more "aggressive" EQ and, to me, an overall warmer, bassier tone. But I've been able to compensate by boosting the bass and cutting the treble control a bit on my guitar and let the Demeter provide the tube sound. It sounded wonderful last night this way and I feel cuts through live mixes well due to a bit more brighter of an overall tone. Hoping to take out a Kern sometime next year to hear it as well.

From what I have gathered, the Demeter is very good if you have a great bass. The Kern's more effective eq section will be able to help a less optimal sounding bass. Of course a mediocre bass will always be and sound that.

I haven't tried the other two, but I have a couple Demeters (the 1 1/2 space and 1-space versions). I play a wide range of active and passive basses through them, and am always impressed by the sound.

While it's true that they don't have an "aggressive" eq, it's more than enough for me. I have an Ampeg SVP-BSP with a much more aggressive EQ section, but my basses just sound sluggish and "thick" through it. I had an Alembic F1x, and although I think it's a fine pre I thought it was even more limited in the "eq" department than the Demeter.